Composite bump flip chip bonding

ABSTRACT

A bonded structure comprising the physical and electrical connections between an integrated circuit element and substrate using a composite bump comprised of a single polymer body of low Young&#39;s Modulus, a conductive barrier metal coating covering the polymer body and a soldering metal coating covering the conductive barrier metal coating. When the bonded structure is formed the composite bump is deformed and the low Young&#39;s Modulus of the polymer body allows a very reliable bonded structure with very low bonding force. Due to the low Young&#39;s Modulus there is little stress tending to break the solder joint after the bonded structure is formed. The bond is formed using a soldering process so that the soldering metal forms a conductive adhesive between the composite bumps and either the substrate input/output pads or the integrated circuit element input/output pads.

RELATED PATENT APPLICATIONS

(1) (E83-0002), U.S. Ser. No. 08/239,375, filed May 6, 1994, entitledComposite Bump Bonding assigned to the same assignee.

(2) (E83-0003), U.S. Ser. No. 08/239,424, filed May 6, 1994, now U.S.Pat. No. 5,393,697, entitled Composite Bump Structure and Methods ofFabrication assigned to the same assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the joining of integrated circuit elements tothe next level of integration and more particularly to the formation ofthe bonded structure which comprises the physical and electricalconnection between the integrated circuit element and the next level ofintegration.

2. Description of the Related Art

In the manufacture of highly dense integrated circuits the formation ofan inexpensive and highly reliable mechanical bond and electricalinterconnection has long been recognized to be of key importance. Sometime ago a solution to this need was patented by L. F. Miller et al inU.S. Pat. No. 3,401,126. This method worked well for many years butincreasing levels of integration and circuit density have made the needfor interconnections on an increasingly fine pitch of key importance.

Flip chip bonding has been done using several types of bumps. One typesimply uses a lead-tin solder or indium alloy solder as the bump whichbonds the integrated circuit chip to a substrate. This type of bondingcan result in shorting between bumps during solder reflow. Another typeof bump uses a copper ball within the lead-tin or indium alloy solder.The copper ball does not melt during solder reflow so there will not bea shorting. However since the copper ball is rigid there can be problemswith cracking of joints. Another type of structure is a stack of solderbumps. This type of structure is not easily shorted during reflow andthe cracking problem is not present. However, it is difficult to makefine pitch solder reflow joints with this method because of stackmisalignment problems.

A method for achieving increased interconnection density was patented byK. Hatada in U.S. Pat. No. 4,749,120. This method employs a gold bump asthe electrical interconnection between the integrated circuit chip andthe substrate while holding the integrated circuit chip in place with aresin coating on the substrate acting as an adhesive between chip andsubstrate. This method has the disadvantage of a Young's Modulus forgold which is very high when compared to that of the resin. As a resultof the Young's Modulus mismatch a very large bonding force is requiredbetween the integrated circuit chip and the substrate during the bumpbonding process while the resin is undergoing its curing cycle. Afterthe bonding process the gold bump will tend to return to its originalshape and the recoil forces will disengage some of the bumps from theelectrodes on the substrate. Another method patented by Y. Tagusa et alin U.S. Pat. No. 4,963,002 employs nickel plated plastic beads or silverparticles to achieve the electrical connection but the former methodsuffers small contact surface area and the latter method has the furtherdisadvantage of the relatively high Young's Modulus for silver.

U.S. Pat. No. 4,916,523 issued to Sokolovsky et al shows aunidirectional conductive adhesive to bond the integrated circuit chipto the substrate. U.S. Pat. No. 5,134,460 issued to Brady et al showsconductive metal bumps covered with a gold layer.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a solder reflow bondedstructure which is not easily shorted during solder reflow, does noteasily crack after solder reflow and which easily accommodates a finepitch solder reflow. This objective is accomplished with a compositebump included in a solder joint. The composite bump includes a singlepolymer body with a low Young's Modulus relative to that of metals and aconductive metal coating covering the polymer body. The bonded structureincludes an integrated circuit element with input/output pads, asubstrate with input/output pads, composite bumps which connect theintegrated circuit element input/output pads and the substrateinput/output pads, and soldering metal which forms the solder joint. Thebonded structure is not easily shorted during a solder reflow bondingprocess, can accommodate a very fine pitch solder reflow easily, anddoes not easily crack after solder reflow is complete. The bondedstructure of this invention forms the electrical and physicalconnections between integrated circuit elements and the correspondingsubstrate wherein very dense wiring patterns can be accommodatedeconomically and the resulting connections are extremely reliable.

It is a further object of this invention to provide a method of forminga bonded structure which includes solder joints and composite bumps.This objective is achieved by forming composite bumps on theinput/output pads of an integrated circuit element or substrate, forminga layer of soldering metal on the composite bumps, bringing theintegrated circuit element and substrate together and heating the solderto solder reflow temperature. Alternatively the objective can beachieved by forming composite bumps on the input/output pads of anintegrated circuit element or substrate, forming a layer of solderingmetal on the input/output pads which do not have the composite bumps,bringing the integrated circuit element and substrate together andheating the solder to reflow temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional representation of composite bumps with asolder layer formed on the integrated circuit element input/output padsbefore soldering.

FIG. 2 is a cross sectional representation of structure after solderingwhere the composite bumps and the solder layer were formed on theintegrated circuit element input/output pads prior to soldering.

FIG. 3 is a cross sectional representation of composite bumps with asolder layer formed on the substrate input/output pads before soldering.

FIG. 4 is a cross sectional representation of structure after solderingwhere the composite bumps and the solder layer were formed on thesubstrate input/output pads prior to soldering.

FIG. 5 is a cross sectional representation of composite bumps formed onthe integrated circuit element input/output pads and a solder layerformed on the substrate input/output pads prior to soldering.

FIG. 6 is a cross sectional representation of the structure aftersoldering where composite bumps were formed on the integrated circuitelement input/output pads and the solder was formed on the substrateinput/output pads prior to soldering.

FIG. 7 is a cross sectional representation of composite bumps formed onthe substrate input/output pads and a solder layer formed on theintegrated circuit element input/output pads prior to soldering.

FIG. 8 is a cross sectional representation of the structure aftersoldering where composite bumps were formed on the substrateinput/output pads and the solder was formed on the integrated circuitelement input/output pads prior to soldering.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer now more particularly to FIGS. 1 and 2, there is shown anembodiment for the bonded structure of the current invention. As shownin FIG. 1 composite bumps are formed on the integrated circuit elementinput/output pads 26 which are a metal such as aluminum about 90 micronsin diameter. Each composite bump is composed of a polymer body 32 suchas polyamic acid polyimide with a thickness between about 5 and 25microns, a conductive metal coating 36 covering the polymer body, and asoldering metal coating 38 covering the conductive metal coating. Theconductive metal coating must adhere to the polymer body and can be acomposite such as chrome/copper/gold, chrome/nickel/gold,chrome/silver/gold, titanium/tungsten/gold, titanium/platinum/gold, ortitanium/palladium/gold. For chrome/copper/gold the thickness can beabout 500 Angstroms chrome/500 Angstroms copper/2000 Angstroms gold. Thesoldering metal layer can be metal alloys such as lead-tin,indium-gallium, or indium-tin. For lead-tin the soldering metal layercan be 95% lead-5% tin with a thickness of between about 1 and 20microns. Substrate input/output pads 24 are formed on the substrate 20and are formed from a metal the solder will wet such aschrome/copper/gold or other metal composites used for the conductivemetal coating on the polymer body of the composite bump. FIG. 2 showsthe bonded structure after soldering. The soldering metal 38 bonds thecomposite bump structure to the substrate input/output pads 24.

FIGS. 3 and. 4 show another embodiment of the bonded structure. As shownin FIG. 3 the composite bumps are formed on the substrate 20input/output pads 24 which are a metal such as aluminum about 90 micronsin diameter. Each composite bump is composed of a polymer body 32 suchas polyamic acid polyimide with a thickness between about 5 and 25microns, a conductive metal coating 36 covering the polymer body, and asoldering metal coating 38 covering the conductive metal coating. Theconductive metal coating must adhere to the polymer body and can be acomposite such as chrome/copper/gold, chrome/nickel/gold,chrome/silver/gold, titanium/tungsten/gold, titanium/platinum/gold, ortitanium/palladium/gold. For chrome/copper/gold the thickness can beabout 500 Angstroms chrome/500 Angstroms copper/2000 Angstroms gold. Thesoldering metal layer can be metal alloys such as lead-tin,indium-gallium, or indium-tin. For lead-tin the soldering metal layercan be 95% lead-5% tin with a thickness of between about 1 and 20microns. Integrated circuit element input/output pads 26 are formed onthe integrated circuit element 30 and are formed from a metal the solderwill wet such as chrome/copper gold or other metal composites used forthe conductive metal coating on the polymer body of the composite bump.FIG. 4 shows the bonded structure after soldering. The soldering metal38 bonds the composite bump structure to the integrated circuit elementinput/output pads 26.

FIGS. 5 and 6 show another embodiment of the bonded structure. As shownin FIG. 5 the composite bumps are formed on the integrated circuitelement input/output pads 26 which are a metal such as aluminum about 90microns in diameter. Each composite bump is composed of a polymer body32 such as polyamic acid polyimide with a thickness between about 5 and25 microns and a conductive metal coating 36 covering the polymer body.The conductive metal coating must adhere to the polymer body and can bea composite such as chrome/copper/gold, chrome/nickel/gold,chrome/silver/gold, titanium/tungsten/gold, titanium/platinum/gold, ortitanium/palladium/gold. For chrome/copper/gold the thickness can beabout 500 Angstroms chrome/500 Angstroms copper/2000 Angstroms gold. Thesoldering metal 38 is formed on the substrate 20 input/output pads 24which must be a metal wettable by the solder, for example copper. Thesoldering metal can be 95% lead-5% tin, other alloys of lead-tin, alloysof indium-gallium, or alloys of indium-tin. FIG. 6 shows the bondedstructure after soldering. The soldering metal 38 bonds the compositebump structure to the substrate input/output pads 24.

FIGS. 7 and 8 show another embodiment of the bonded structure. As shownin FIG. 7 the composite bumps are formed on the substrate 20input/output pads 24 which are metal such as aluminum and about 90microns in diameter. Each composite bump is composed of a polymer body32 such as polyamic acid polyimide with a thickness between about 5 and25 microns and a conductive metal coating 36 covering the polymer body.The conductive metal coating must adhere to the polymer body and can bea composite such as chrome/copper/gold, chrome/nickel/gold,chrome/silver/gold, titanium/tungsten/gold, titanium/platinum/gold, ortitanium/palladium/gold. For chrome/copper/gold the thickness can beabout 500 Angstroms chrome/500 Angstroms copper/2000 Angstroms gold. Thesoldering metal 38 is formed on the integrated circuit element 30input/output pads 26 which must be a metal wettable by the solder, forexample chrome/copper/gold or other composites used as the conductivemetal coating covering the polymer body. The soldering metal can be 95%lead-5% tin, other alloys of lead-tin, alloys of indium-gallium, oralloys of indium-tin. FIG. 8 shows the bonded structure after soldering.The soldering metal 38 bonds the composite bump structure to theintegrated circuit element input/output pads 26.

FIGS. 1 and 2 show an embodiment of a method of forming the bondedstructure of the current invention. As shown in FIG. 1 composite bumpsare formed on the integrated circuit element input/output pads 26 whichare a metal such as aluminum. Each composite bump is composed of apolymer body 32 such as polyamic acid polyimide, a conductive metalcoating 36 covering the polymer body, and a soldering metal coating 38covering the conductive metal coating. The conductive metal coating mustadhere to the polymer body and can be a composite such aschrome/copper/gold, chrome/nickel/gold, chrome/silver/gold,titanium/tungsten/gold, titanium/platinum/gold, ortitanium/palladium/gold. For chrome/copper/gold the thickness can beabout 500 Angstroms chrome/500 Angstroms copper/2000 Angstroms gold. Thesoldering metal layer 38 can be metal alloys such as lead-tin,indium-gallium, or indium-tin. For lead-tin the soldering metal layercan be 95% lead-5% tin with a thickness of between about 1 and 20microns. Substrate input/output pads 24 are formed on the substrate 20and are formed from a metal the solder will wet such chrome/copper/goldor other metal composites used for the conductive metal coating on thepolymer body of the composite bump.

The integrated circuit element 30 and the substrate 20 are then broughttogether so that the composite bumps are brought together with thesubstrate input/output pads 24. Heat is applied to the soldering metal38 thereby raising its temperature to about 30° C. above the meltingpoint of the soldering metal used. For the 95% lead-5% tin solder alloythe temperature is raised to about 350° C. The soldering metal meltswetting the substrate input/out pads 24, the heat is removed, and as thesolder cools below the melting point the bonded structure is formed. Dueto the low Young's Modulus of the polymer body the stress tending tobreak the solder joint during or after the soldering process isextremely small.

FIGS. 3 and 4 show another embodiment of a method of forming the bondedstructure. As shown in FIG. 3 the composite bump is formed on thesubstrate 20 input/output pad 24 prior to bonding. The composite bumpincludes a polymer body 32, a conductive metal coating 36, and asoldering metal coating 38 such as alloys of lead-tin, indium-gallium,or indium-tin. The integrated circuit element 30 input/output pad 26 isformed from a metal wettable by the solder used.

The integrated circuit element 30 and the substrate 20 are then broughttogether so that the composite bumps are brought together with theintegrated circuit element 30 input/output pads 24. Heat is applied tothe soldering metal 38 thereby raising its temperature to about 30° C.above the melting point of the soldering metal used. For the 95% lead-5%tin solder alloy the temperature is raised to about 350° C. Thesoldering metal melts wetting the integrated circuit element input/outpads 26, the heat is removed, and as the solder cools below the meltingpoint the bonded structure is formed. Due to the low Young's Modulus ofthe polymer body the stress tending to break the solder joint during orafter the soldering process is extremely small.

FIGS. 5 and 6 show another embodiment of a method of forming the bondedstructure. As shown in FIG. 5 the composite bump is formed on theintegrated circuit element 30 input/output pads 26 prior to bonding. Thecomposite bump includes a polymer body 32 and a conductive metal coating36. A soldering metal coating 38, such as alloys of lead-tin,indium-gallium, or indium-tin, is formed on the substrate 20input/output pads 24. The substrate 20 input/output pads 24 are formedfrom a metal wettable by the solder used. The integrated circuit element30 and the substrate 20 are then brought together so that the compositebumps are brought together with the substrate 20 input/output pads 24.The soldering process then proceeds as described in the previousembodiments.

FIGS. 7 and 8 show another embodiment of a method of forming the bondedstructure. As shown in FIG. 7 the composite bump is formed on thesubstrate 20 input/output pads 24 prior to bonding. The composite bumpincludes a polymer body 32 and a conductive metal coating 36. Asoldering metal coating 38, such as alloys of lead-tin, indium-gallium,or indium-tin, is formed on the integrated circuit element 30input/output pads 26. The integrated circuit element 30 input/outputpads 26 are formed from a metal wettable by the solder used. Theintegrated circuit element 30 and the substrate 20 are then broughttogether so that the composite bumps are brought together with theintegrated circuit element 30 input/output pads 26. The solderingprocess then proceeds as described in the previous embodiments.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for forming a bonded structure,comprising:providing an integrated circuit element with input/outputpads; providing a single polymer body on each said integrated circuitelement input/output pad wherein the cross section area of said singlepolymer body on said integrated circuit element input/output pad issmaller than that of said integrated circuit element input/output pad;providing a conductive metal coating upon said single polymer body andcovering said integrated circuit element input/output pad wherein eachsaid single polymer body and said coductive metal coating comprises acomposite bump; providing a substrate with input/output pads; providinga soldering metal; bringing together said composite bumps formed on saidintegrated circuit element input/output pads, said substrateinput/output pads, and said soldering metal; heating said solderingmetal to a temperature of about 30° C. above the melting point of saidsoldering metal; and cooling said soldering metal below the meltingpoint of said soldering metal.
 2. The method of claim 1 wherein saidpolymer is polyamic acid polyimide.
 3. The method of claim 1 whereinsaid conductive metal coating is a composite of 500 Angstroms chrome/500Angstroms copper/2000 Angstroms gold and said conductive metal coatingand said single polymer body of said composite bumps have such materialand thickness characteristics that said composite bumps are permanentlydeformed and crushed into permanent physical and electrical contact withsaid substrate input/output pads.
 4. The method of claim 1 wherein saidsoldering metal is 95% lead-5% tin.
 5. The method of claim 1 whereinsaid soldering metal is formed on said composite bumps prior to heatingsaid soldering metal.
 6. The method of claim 1 wherein said solderingmetal is formed on said substrate input/output pads prior to heatingsaid soldering metal.
 7. A method for forming a bonded structure,comprising:providing a substrate with input/output pads; providing asingle polymer body on each said substrate input/output pad, wherein thecross section area of said single polymer body on said substrateinput/output pad is smaller than that of said integrated circuit elementinput/output pad; providing a conductive metal coating upon said singlepolymer body and covering said substrate input/output wherein each saidsingle polymer body and said conductive metal coating comprises acomposite bump; providing an integrated circuit element withinput/output pads; providing a soldering metal; bringing together saidcomposite bumps formed on said substrate input/output pads, saidintegrated circuit element input/output pads, and said soldering metal;heating said soldering metal to a temperature of about 30° C. above themelting point of said soldering metal; and cooling said soldering metalbelow the melting point of said soldering metal.
 8. The method of claim7 wherein said polymer is polyamic acid polyimide.
 9. The method ofclaim 7 wherein said conductive metal coating is a composite of 500Angstroms chrome/500 Angstroms copper/2000 Angstroms gold and saidconductive metal coating and said single polymer body of said compositebumps have such material and thickness characteristics that saidcomposite bumps are permanently deformed and crushed into permanentphysical and electrical contact with said integrated circuit elementinput/output pads.
 10. The method of claim 7 wherein said solderingmetal is 95% lead-5% tin.
 11. The method of claim 7 wherein saidsoldering metal is formed on said composite bumps prior to heating saidsoldering metal.
 12. The method of claim 7 wherein said soldering metalis formed on said integrated circuit element input/output pads prior toheating said soldering metal.